The answer would be a planet<span>. Planets revolve around stars, which means there will come a point where the planet is between the star and our field of vision towards the star. This point will be where the star's radiation will have the lowest intensity. As the planet moves, the intensity will change. The effect is comparable to a lunar or solar eclipse.</span>
If a chain reaction takes place, the amount of energy released would increase exponentially, so in order to control the energy release, fission must be controlled without controlling a chain reaction, so the third option is correct.
